Particulate matter

Exhaust rises from smokestacks in front of piles of coal at NRG Energy's W.A. Parish Electric Generating Station in Texas in 2011. The Environmental Protection Agency proposed new rules Thursday that would lower the amount of fine particulate matter, or soot, that enters the air from sources...

Exhaust rises from smokestacks in front of piles of coal at NRG Energy's W.A. Parish Electric Generating Station in Texas in 2011. The Environmental Protection Agency proposed new rules Thursday that would lower the amount of fine particulate matter, or soot, that enters the air from sources... (David J. Phillip / Associated Press)

Exhaust rises from smokestacks in front of piles of coal at NRG Energy's W.A. Parish Electric Generating Station in Texas in 2011. The Environmental Protection Agency proposed new rules Thursday that would lower the amount of fine particulate matter, or soot, that enters the air from sources...